Strange coloration & lines on the monitor

I was called in to take a look at a 5 yr old Gateway Performance 600, P-III,
loaded w/ Windows 98se. I got the serial # and got the list from Gateway
for the original package. I went prepared and took a new video card thinking
it's probably the problem. It belongs to a 60 Yr old woman that doesn't really
use it much. But she does turn it on and leave it on all day.
The Monitor has strange lines (like horizontal bands of darker color).
I replaced the video card, no change. I tried 2 different monitors, still
the same. Oh, I brought it home and hooked it up to one of mine, it's
the same, so it's not electrical/magnetic field interference.

Yes, everything on a surge protector. It was Windows 98 ( no option of refresh rate ). I took parts from the Gateway and used another machine and made her an upgrade.
But when I would try to do anything w/ the dial up modem after the 1st attempt it would go to blue screen and give me a bios problem warning.
I put in another modem and now no problem.
It must have been the culprit!

IS it Vertical lines lllll black and grey? Theres a way to fix that it happened to me when syncing my monitor to a setting that should have worked but didnt lol. Later on I had to find a way to fix the problem. I cant think of it atm the moment but nothing should be wrong cause im still using my monitor that had a simaliar problem. Im also using the settings that I was trying to use before... ER I had this problem in 2003 if this sounds simliliar . Give me full details on your problems from games to programs you installed to video crad drivers to direct X. Syncing your lcd while direct x test is going on can cause the problem also with the verical grey and black lines stuff.

It's really a moot point now, other than just wanting answers.
I took some of the higher end components
out of the Gateway and put them in a Dell optiplex I was rebuilding.
I'm 99% sure it was the dial up modem that was causing the problem.
The Horizontal ghostly shadow like lines disappeared when I replaced the
modem.